Background: It seems that the sexual cycle is highly sensitive to environmental light-dark conditions; i.e., halted and irregulated during short-day and long-day conditions, respectively. These effects are mediated by the pineal gland, which converts light-dark signals into a chemical signal (melatonin [Mel]). Objective: The aims of this study were firstly, to evaluate the effects of long day (short night) conditions on the regularity of estrus cycle and the probable spatial working memory deficits and secondly, the curing effects of oral administration of ML on the above parameters. Materials and Methods: A total of 30 female Wistar rats (3-4 month, 220 ± 30 gr) were randomly divided into 6 groups (n = 5/each). Experimental groups were categorized as control, control + Mel, experimental 1, experimental 1 + Mel, experimental 2, and experimental 2 + Mel. Light conditions included standard light: dark (12 hr light/ dark cycle), short night (16 hr light/8 hr dark), and long night (20 hr light/4 hr dark) with free access to food and water was provided. They were exposed to different light conditions (light-dark cycles) for 6 wk. Mel was administered at a final concentration of 0.8 µg/mL dissolved in drinking water. Control groups received a placebo (ethanol). To check the regularity of the sexual cycle daily vaginal smear cytology was done. During the final week, elevated plus maze test was undertaken to assess the probable spatial working memory deficit. Results: Female rats kept in long light conditions show severe irregularity in estrus cycles and Mel administration significantly reduced the number of irregular cycles (p = 0.0006). Exposure to night-time light and Mel treatment had no obvious effect on spatial working memory. Conclusion: Based on the results obtained from the present study, it can be concluded that Mel administration may correct the disturbances of the sexual cycles via its strong anti-oxidant properties.

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